Piezoelectric materials have great potentials for applications in a variety of micro electromechanical system (MEMS) devices, such as micro sensors, microactuators, micropumps, piezoelectric ink-jet printing heads, etc. In many of these MEMS devices, piezoelectric materials such as piezoelectric thin films are formed on one side of a substrate and the microstructures including a plurality of recesses are formed on the opposite side of the substrate.
A number of methods are used in the art to form piezoelectric thin films, among which the solution spin-coating process is widely used for preparing inorganic piezoelectric films with complex oxide composition and piezoelectric polymers. Inorganic piezoelectric film with complex oxide composition has the best piezoelectric performance than other reported thin films. Solution spin-coating process, including multiple solution spin-coating followed by multiple heat treatment, such as sol-gel and Metallo Organic Decomposition (MOD), possesses great advantages of precise composition control and low cost in obtaining the high performance piezoelectric complex oxide films.
In a conventional method of producing a piezoelectric MEMS device by solution spin-coating process, a piezoelectric thin film is firstly deposited on one side of the substrate, and a desired microstructure is then formed at the opposite side of the substrate by an etching process, such as the wet-etching process. Before the etching process, the piezoelectric films formed on the substrate need to be temporarily protected, so that the highly corrosive etching solution can be prevented from contacting and damaging the piezoelectric films.
There are usually two methods in the art to protect the piezoelectric films during fabrication of the MEMS devices in the etching process. One method is to deposit a thick coating of protective material, such as photosensitive polyimide, wax or the like, onto the piezoelectric films prior to the etching process. Another method is a type of a mechanical protection by using a protective jig to cover the piezoelectric films prior to the etching process.
Both the two methods have drawbacks. In the first method, if the coating is not tightly encapsulated on the piezoelectric films, the etchant may leak into the coating hence damage and/or contaminate the piezoelectric films. In addition, since the coating is formed for the temporarily protection purpose, after the etching process, the coating needs to be removed and therefore, it consumes time, material and labor hence the cost of production is high. Further, the removal of the coating may cause contamination and damage to the piezoelectric films. In the second method, the protective jig may also encounter potential leakage problem due to any imperfect sealing of the protective jig. In addition, since the protective jig needs to be mechanically attached to the piezoelectric film or the substrate, the stress induced by the mechanical attachment may damage the substrate. Moreover, the productivity of this process is very low.
It is therefore a need to provide a method and apparatus of fabricating piezoelectric membrane structure for MEMS devices, in which the piezoelectric film formed on the substrate is well protected from being damaged and/or contaminated during the fabrication process. In the meantime, there is also a need to increase the yield and productivity of micromachined piezoelectric device fabrication.